Self-Heating Mould for Composite Manufacturing

Abstract

The shipbuilding industry, engine manufacturing, aviation, rocket and space technology are promising fields of application for polymeric composite materials. Shape-generating moulding tools with internal heating are used for the creation of a more economically viable method of moulding of internally heated composite structures. The use of a fine-fibered resistive structure in the heated tools allows implementation of effective heating of the composite and elimination of the need for expensive and energy-intensive heating equipment. The aim of this paper was the reduction of energy consumption for internally heated moulding tools by choosing the optimal parameters for their resistive layer. A method for determination of the parameters of the moulding tool resistive layer was developed. This method allows calculation of the heating layer parameters and implementation of the specified time–temperature regime for moulding of the composite structure. It was shown that energy saving for the heated fiberglass shape-generating moulding tools was from 40 to 60%. It was found that the increase in the thickness of the moulded package of the polymeric composite material resulted not only in a higher supplied power for the heating system, but also in a complication of the method for system control, because of the growing exothermic effect of the binder curing reaction. For composite products based on Hysol EA 9396 binder, thicknesses more than 4 mm are critical, because it is not possible to cope with the self-heating effect only by cooling with ambient air already utilized at the twentieth minute of the moulding process. The influence of the physical and mechanical characteristics of the moulding tool material and stiffening ribs was analysed in terms of energy consumption and controllability of the heating system. Fiberglass shows the lowest energy consumption. Heating of the aluminium and steel moulding tools for the same purpose will require 20% and 45% more power, respectively. An increase in the number of stiffening ribs has a strong effect on the heat removal of the heating system. With a small number of aluminium ribs it is not possible to maintain the specified temperature–time regime for a fiberglass moulded package of 5 mm thick with the use of the equipment. However, when the number of stiffeners is increased to 10, the exothermic effect of the reaction becomes smoother and then the heating equipment can cope with the task. An experimental prototype of heating equipment of moulding tools for the manufacturing of structures of polymeric composite materials, as well as a flexible thermal blanket for repair of non-separable structures, were developed. The results can be the basis for a new method of optimal design of parameters of moulding tool structure at minimal heat removal to the environment.

Effects of the Temperature–Time Regime of Curing of Composite Patch on Repair Process Efficiency

“…In order to determine the stress-strain behaviour of the structure under repair, we apply the analytical model of a panel of step-variable thickness [19]. Now we shall analyse the external and internal force factors leading to the appearance of stresses in the structure under repair, using the example of typical temperature-time dependence of the polymeric binder curing mode (Figure 3) [19,22].…”

“…the external and internal force factors leading to the appearance of stresses in the structure under repair, using the example of typical temperature-time dependence of the polymeric binder curing mode (Figure 3) [19,22]. Taking into account that technological stresses occur in the structure only after gelation (point O) [25,29,31], the typical curing process can be roughly divided into four stages, as shown in Figure 3.…”

“…At the first stage, occurrence and growth of stresses is determined by heating and available shrinkage deformations. When metal structures are repaired using the composite patch, thermal and shrinkage stresses can have the same sign at the definite ratio of the linear thermal expansion coefficient and stiffnesses [19]. In this case, temperature stresses supplemented with shrinkage ones may lead to occurrence of defects already in the heating process, since physico-mechanical characteristics of the patch material at the first stage are considerably lower than the final values.…”

“…In order to determine the stress-strain behaviour of the structure with the characteristics changing in the curing process, arising stresses are determined as a sum of stresses on the certain areas, within which [19]:…”

“…However, stresses occurring in the process of curing the repair patch further affect the bearing capacity of repaired product [16]. Presence of unrecorded initial stresses in the repaired structure causes the appearance of defects such as delaminations, buckling and microcracks in the binder [17,18], which in turn has a significant effect on the static and fatigue strength of the product made with such structures [19,20].…”